^ Peter Ladefoged (1964) A Phonetic Study of West African Languages (in English), Cambridge: Cambridge University Press, page 25: “The term approximant is used here to describe a sound which belongs to the phonetic class vocoid or central resonant oral (Pike, 1943), and simultaneously to the phonological class consonant in that it occurs in the same phonotactic patterns as stops, fricatives and nasals.”

^ Philip J. Maher (2017) Operator Approximant Problems Arising from Quantum Theory (in English), →ISBN, page 1: “The key concept of this book is that of an approximant (the characteristically snappy term is due to Halmos [21])”